Resistance to Sliding in Clear and Metallic Damon 3 and Conventional Edgewise Brackets: an In vitro Study.

STATEMENT OF THE PROBLEM
Frictional forces are considered as important counterforce to orthodontic tooth movement. It is claimed that self-ligating brackets reduce the frictional forces.


PURPOSE
The aim of this study was to compare the resistance to sliding in metallic and clear Damon brackets with the conventional brackets in a wet condition.


MATERIALS AND METHOD
The samples included 4 types of brackets; metallic and clear Damon brackets and metallic and clear conventional brackets (10 brackets in each group). In this study, stainless steel wires sized 0.019×0.025 were employed and the operator's saliva was used to simulate the conditions of oral cavity. The tidy-modified design was used for simulation of sliding movement. The resistance to sliding and static frictional forces was measured by employing Testometric machine and load cell.


RESULTS
The mean (±SD) of resistance to sliding was 194.88 (±26.65) and 226.62 (±39.9) g in the esthetic and metallic Damon brackets, while these values were 187.81(±27.84) and 191.17(±66.68) g for the clear and metallic conventional brackets, respectively. Static frictional forces were 206.4(±42.45) and 210.38(±15.89) g in the esthetic and metallic Damon brackets and 220.63(±49.29) and 215.13(±62.38) g in the clear and metallic conventional brackets. According to two-way ANOVA, no significant difference was observed between the two bracket materials (clear and metal) and the two types of bracket (self-ligating versus conventional) regarding resistance to sliding (p= 0.17 and p= 0.23, respectively) and static frictional forces (p= 0.55 and p= 0.96, respectively).


CONCLUSION
Neither the type of bracket materials nor their type of ligation made difference in resistance to sliding and static friction.


Introduction
Sliding a tooth along an arch wire is a very common orthodontic procedure to translate tooth, especially during the closure of spaces in extraction cases and correction of dental irregularities. [1] The advantages of this technique are shorter clinical treatment time, more patient convenience, and better controlling of three dimensional tooth movements. [2] On the other hand, one of the disadvantages of this system is the frictional forces between wire and brackets. These forces can result in decreased treatment efficiency, loss of anchorage, and consequently unwanted tooth movement. [3] Two major types of friction can be defined as static frictional force which is the smallest force needed to start a motion of solid surface, and kinetic frictional force which is the force required to resist the sliding motion of one solid object over another at a constant speed. Most studies use the term friction as a static frictional force that equals coefficient of frictional force multiply by forces perpendicular to line of motion (perpendicular to wire). [4] During sliding mechanism in orthodontic treatment, a part of the applied force is dissipated to overcome friction, while friction is transmitted to the tooth supporting structure inducing tooth movement. [5] It was reported that 12% to 60 % of the force induced by fixed orthodontic appliances are used to overcome friction. [6] Combinations of mechanical and chemical factors are determinant of friction between wires and brackets. [7] Some studies have investigated the factors associated with friction between wires and brackets and have listed them as clearance between wires and brackets, size of wire, cross section of the wire (round or rectangular), incorporated torque in bracket and wire, area of cross section of wire and bracket slot, wire and brackets materials, width of bracket slot, type of bracket (conventional versus self-ligate), type and amount of ligation force, [8] in addition to environmental condition such as temperature and presence of lubricant. [9] Each intermediate material that reduces contact area between two surfaces can be used as a lubricant or antifriction substance. [10] There are lots of controversies about the role of saliva in friction. Some studies have mentioned that saliva can reduce friction, while others express the opposite. [10] So far, few studies have assessed the effect of natural saliva in reducing or increasing the friction, and most of them have used artificial saliva in their investigations. [11] Friction in clinical orthodontics is now receiving more attention because orthodontic companies have proclaimed that low friction was good, and the concept was applied for marketing their self-ligating brackets. [12] The Damon SL bracket is a self-ligating bracket which does not exert spring pressure on the arch wire, and uses covers which slide vertically in an occlusal direction. [10] The slot size of these brackets is 0.022×0.027 inch 2 . [1] Nowadays, clear self-ligating brackets have become very popular in orthodontic practice and both the patients and orthodontists are more interested in using them. Since there is not enough evidence in regard to frictional forces in these types of brackets, the present study was designed. The aim of this study is to investigate and compare the amount of force resistance to sliding (combination of frictional force, binding, and notching) [12] in metallic and clear conventional MBT and Damon brackets in wet condition (natural saliva). To the best of authors' knowledge, no study has been performed on clear Damon or selfligating brackets, so far.

Materials and Method
The study was done in Hamadan School of Dentistry and Amirkabir University. We used Damon 3MX  brackets. The conventional brackets that were fixed to base plate were tied with ligature wire, while movable brackets were tied according to their self-ligating system and conventional brackets were tied with ligature wire.
Oral cavity condition was simulated with un-stimulated saliva of the operator who had orthodontic appliance in his mouth. Saliva was applied with dropper to arch wire for each examination. The movable bracket was attached to the center of a brass bar with a cyanoacrylate and a 100 g weight was hung at a 10 mm distance from the center of bracket in order to represent equivalent single force acting on the resistance center of the tooth.
The entire test was done using Testometric machine (220 D; Testometric co., unit 1, Lancashire, UK) with a cross head speed of 5 mm/min ( Figure 2). The movable bracket was hung to the load cell throughout the test. It is necessary to mention that before performing each examination and hanging the weights, the machine was calibrated to ignore the weight of movable bracket and brass bar. In each examination, after suspending the weights, movable brackets passed a distance more than 10 mm and the load cell recorded the force value. The force value recorded with load cell can be defined as the clinical force required for retracting a canine, while the difference between this value and the weights can be defined as a frictional force. The measurements and information from examinations were converted to graph using a software (Testometric's feature-rich winTest™ Analysis software). In the graph, the first peak represents the static frictional force, and 7mm after the peak represents the value of resistance to sliding. Mean and standard deviation were measured for each bracket.
Two-way ANOVA test was used for statistical analysis with respect to the type of brackets (clear versus metallic) and ligation (self-ligating versus conventional). The level of significant was set at p< 0.05.

Results
The mean (±SD) of resistance to sliding was 194.88 With respect to statistical analysis, bracket material (ceramic versus metallic) had no effect on resistance to sliding and static friction in the current study. One of the  In this study, two-way ANOVA test revealed no significant difference between the two types of bracket (self-ligating versus conventional) regarding resistance to sliding (p= 0.23) and static frictional forces (p= 0.96).
The studies [4,10,[21][22][23][24][25] can be divided into two groups based on the resistance force to sliding and static friction in Damon and conventional brackets. The first group [21][22][23][24] reported no difference between selfligating and conventional brackets, while the second group [4,10,25] claimed that self-ligating brackets produce less friction than conventional ones. [10] used rigid bar with one fixed bracket, and pulled the wire from the bracket. They also used the drawing force for friction analysis. As it is known, this drawing force is combination of the force required to move the wire and the frictional forces. In our study, a removable bracket was used that slid along the wire; furthermore, a weight was hung at a 10 mm distance from the center of bracket to represent equivalent single force acting on the resistance center of the tooth. This design was more similar to the real sliding mechanism used in clinical practice. In the present study, unlike the above mentioned studies, the precise friction value can be identified by subtracting the weight value from the value obtained with testing machine. As mentioned by Clocheret et al., most researchers have used different protocols or even approaches to evaluate the friction generated in different wire-bracket combinations. [15] Thus, the published results of many studies are difficult to compare. Using an in-vitro environment was probably a major limitation of this study; hence, these results should be used with caution due to the apparent difference between oral and in-vitro environment.

Conclusion
There was no significant difference between clear and metallic Damon and conventional brackets regarding resistance to sliding and static friction, in wet condition on a 0.019×0.025 SS wire. In full-size archwires, there was a small difference between various bracket types and materials.